U.S. patent number 6,908,496 [Application Number 10/335,993] was granted by the patent office on 2005-06-21 for method for scalable production of nanoshells using salt assisted purification of intermediate colloid-seeded nanoparticles.
This patent grant is currently assigned to William Marsh Rice University. Invention is credited to Robert Kelley Bradley, Nancy J. Halas.
United States Patent |
6,908,496 |
Halas , et al. |
June 21, 2005 |
Method for scalable production of nanoshells using salt assisted
purification of intermediate colloid-seeded nanoparticles
Abstract
A method for purifying a suspension containing colloid-seeded
nanoparticles and excess colloids is provided that includes adding
to the suspension a filter aid comprising a salt. The method
further includes filtering the suspension with a filter of a pore
size intermediate between the average colloid-seeded nanoparticle
size and the average excess colloid size, so as to form a retentate
that includes the majority of the colloid-seeded nanoparticles and
a filtrate that includes the majority of the excess colloids. Still
further, the method includes collecting the retentate. The method
may be incorporated into a method of making metallized
nanoparticles, such as nanoshells, by reduction of metal ions onto
the purified colloid-seed nanoparticles so as to form the
metallized nanoparticles.
Inventors: |
Halas; Nancy J. (Houston,
TX), Bradley; Robert Kelley (Houston, TX) |
Assignee: |
William Marsh Rice University
(Houston, TX)
|
Family
ID: |
27807757 |
Appl.
No.: |
10/335,993 |
Filed: |
January 2, 2003 |
Current U.S.
Class: |
75/370; 210/639;
210/650; 210/651; 75/371 |
Current CPC
Class: |
B22F
1/025 (20130101); B82Y 30/00 (20130101); B22F
1/0022 (20130101) |
Current International
Class: |
B22F
1/02 (20060101); B22F 1/00 (20060101); B01D
061/14 () |
Field of
Search: |
;75/342,370,371
;210/639,650,651 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Merriam-Webster's Collegiate Dictionary, Tenth Edition, p. 1032,
Merriam-Webster Incorporated, 1999. .
John C. Crocker; Measurement of the Hydrodynamic Corrections to the
Brownian Motion of Two Colloidal Spheres; J. Chem. Phys. 106(7)
Feb. 15, 1997; (pp. 2837-2840)..
|
Primary Examiner: Wyszomierski; George
Attorney, Agent or Firm: Conley Rose, P.C.
Government Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
This work was supported by funding from the Army Grant Number DAAD
19-99-1-0315.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of provisional application Ser.
No. 60/345,714, filed Jan. 2, 2002, entitled "Method for Large
Scale Production of Nanoshells Using Salt-Assisted Purification of
Intermediate Colloid-Seeded Nanoparticles", hereby incorporated
herein by reference.
Claims
What is claimed is:
1. A method for purifying a suspension containing colloid-seeded
nanoparticles and excess colloids, comprising: (a) adding to the
suspension a filter aid comprising a salt; (b) filtering the
suspension with a filter of a pore size intermediate between the
average colloid-seeded nanoparticle size and the average excess
colloid size, so as to form a retentate comprising the majority of
the colloid-seeded nanoparticles and a filtrate comprising the
majority of the excess colloids, wherein each colloid-seeded
nanoparticle comprises a plurality of linkers, and a plurality of
colloids bound to a portion of the linkers, the remaining portion
of the linkers being free; (c) collecting the retentate; and (d)
adding a plurality of protectants to the retentate so as to
passivate the free linkers.
2. The method according to claim 1 wherein step (b) comprises: (b1)
loading the suspension to a retentate zone, wherein the filter
separates the retentate zone from a filtrate zone; and (b2) passing
a first wash liquid through the retentate zone.
3. The method according to claim 2 wherein step (b2) comprises:
passing the first wash liquid tangentially to the filter such that
the filtering comprises crossflow filtering.
4. The method according to claim 2 wherein the first wash liquid
comprises a solution of the salt added in step (a), wherein the
solution comprises the salt in a concentration about equal to the
salt concentration of the suspension after step (a).
5. The method according to claim 2 wherein the first wash liquid
comprises a surfactant.
6. The method according to claim 5 wherein the surfactant comprises
polyethylene glycol.
7. The method according to claim 2 wherein step (b) further
comprises: (b3) passing a second wash liquid through the retentate
zone.
8. The method according to claim 7 wherein the second wash liquid
comprises water.
9. The method according to claim 1 wherein each linker comprises an
amine moiety and wherein each protectant comprises 1-ethyl
-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC).
10. The method according to claim 1 wherein the salt comprises
sodium chloride.
11. A method for treating a suspension containing colloid-seeded
nanoparticles and excess metal colloids comprising: (a) adding to
the suspension a filter aid comprising a salt; (b) filtering the
suspension with a filter of a pore size intermediate between the
average seeded nanoparticle size and the average excess colloid
size, so as to form a retentate comprising the majority of the
colloid-seeded nanoparticles and a filtrate comprising the majority
of the excess colloids, wherein each colloid-seeded nanoparticle
comprises free linkers, and further wherein the filtering
comprises: (b1) loading the suspension to a crossflow filtration
zone containing the filter; and (b2) passing a first wash liquid
through the filtration zone tangentially to the filter; (c)
collecting the retentate; and (d) adding a plurality of protectants
to the retentate so as to passivate the free linkers.
12. The method according to claim 11 further comprising: (e)
reducing metal ions onto the seeded nanoparticles so as to form
metallized nanoparticles.
13. The method according to claim 11 wherein each metal colloid
comprises gold.
14. The method according to claim 13 wherein the gold comprises
filtrate gold.
15. The method according to claim 11 wherein the metal ions are
selected from the group consisting of gold ions, silver ions,
platinum ions, palladium ions, nickel ions, iron ions, and copper
ions.
16. A method for making metallized nanoparticles via intermediate
colloid-seeded nanoparticles, comprising: (a) providing a
suspension of the colloid-seeded nanoparticles, wherein the
colloids comprise a first metal, and further wherein each
colloid-seeded nanoparticle comprises free linkers; (b) adding to
the suspension a filter aid comprising a salt; (c) filtering the
suspension with a filter of a pore size intermediate between the
average seeded nanoparticle size and the average excess colloid
size, so as to form a retentate comprising the majority of the
seeded nanoparticles and a filtrate comprising the majority of the
excess colloids; (d) collecting the retentate and adding a
plurality of protectants to the retentate so as to passivate the
free linkers; and (e) reducing ions of a second metal onto the
colloid-seeded nanoparticles so as to form the metallized
nanoparticles.
17. The method according to claim 16 wherein step (b) comprises:
(b1) loading the suspension to a retentate zone, wherein the filter
separates the retentate zone from a filtrate zone; and (b2) passing
a first wash liquid through the retentate zone.
18. The method according to claim 17 wherein step (b2) comprises:
passing the first wash liquid tangentially to the filter such that
the filtering comprises crossflow filtering.
19. The method according to claim 17 wherein the first wash liquid
comprises a solution of the salt added in step (a), wherein the
solution comprises the salt in a concentration about equal to the
salt concentration of the suspension after step (a).
20. The method according to claim 17 wherein the first wash liquid
comprises a surfactant.
21. The method according to claim 20 wherein the surfactant
comprises polyethylene glycol.
22. The method according to claim 17 wherein step (b) further
comprises: (b3) passing a second wash liquid through the retentate
zone.
23. The method according to claim 22 wherein the second wash liquid
comprises water.
24. The method according to claim 16 wherein step (a) comprises
mixing a suspension of colloids with a suspension of linker-coated
nanoparticles so as to form the suspension of the colloid-seeded
nanoparticles.
25. The method according to claim 16 wherein each linker comprises
an amine moiety and each protectant comprises
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride
(EDC).
26. The method according to claim 16 wherein the second metal has
the same identity as the first metal.
27. The method according to claim 16 wherein the second metal has
an identity different from the first metal.
28. The method according to claim 16 wherein the first metal
comprises gold.
29. The method according to claim 28 wherein the gold comprises
filtrate gold.
30. The method according to claim 16 wherein the second metal is
selected from the group consisting of gold, silver, platinum,
palladium, nickel, iron, and copper.
31. The method according to claim 16 wherein the salt comprises
sodium chloride.
Description
FIELD OF THE INVENTION
The present invention relates generally to a method for scalable
production of metallized nanoparticles, such as metal nanoshells.
More particularly, the present invention relates to a method that
uses salt-assisted filtration to separate excess metal colloid from
intermediate colloid-seeded nanoparticles so as to purify the
colloid-seeded nanoparticles.
BACKGROUND OF THE INVENTION
Particles able to absorb or scatter light of well-defined colors
have been used in applications involving detection, absorption, or
scattering of light, for example medical diagnostic imaging. Such
particles are typically colloidal metal particles. The term
colloidal conventionally refers to the size of the particles,
generally denoting particles having a size between about 1
nanometer and about 1 micron.
Small particles made from certain metals that are in the size range
of colloidal metal particles tend to have a particularly strong
interaction with light, termed a resonance, with a maximum at a
well-defined wavelength. Such metals include gold, silver,
platinum, and, to a lesser extent, others of the transition metals.
Light at the resonance wavelength excites particular collective
modes of electrons, termed plasma modes, in the metal. Hence the
resonance is termed the plasmon resonance.
By selecting the metal material of a colloidal particle, it
possible to vary the wavelength of the plasmon resonance. When the
plasmon resonance involves the absorption of light, this gives a
solution of absorbing particles a well-defined color, since color
depends on the wavelength of light that is absorbed. Solid gold
colloidal particles have a characteristic absorption with a maximum
at 500-530 nanometers, giving a solution of these particles a
characteristic red color. The small variation in the wavelength
results from a particle size dependence of the plasmon resonance.
Alternatively, solid silver colloidal particles have a
characteristic absorption with a maximum at 390-420 nanometers,
giving a solution of these particles a characteristic yellow
color.
Using small particles of various metals, particles can be made that
exhibit absorption or scattering of selected characteristic colors
across a visible spectrum. However, a solid metal colloidal
particle absorbing in the infrared is not known. Optical
extinction, in particular absorption or scattering, in the infrared
is desirable for imaging methods that operate in the infrared.
Further, optical communications, such as long distance phone
service that is transmitted over optical fibers, operate in the
infrared.
It has been speculated since the 1950's that it would be
theoretically possible to shift the plasmon resonance of a metal to
longer wavelengths by forming a shell of that metal around a core
particle made of a different material. In particular, the full
calculation of scattering from a sphere of arbitrary material was
solved by Mie, as described in G. Mie, Ann. Phys. 24, 377 (1908).
This solution was extended to concentric spheres of different
materials, using simplifying assumptions regarding the dielectric
properties of the materials, by Aden and Kerker, as described in A.
L. Aden and M. Kerker, J. of Applied Physics, 22, 10, 1242 (1951).
The wavelength of the plasmon resonance would depend on the ratio
of the thickness of the metal shell to the size, such as diameter
of a sphere, of the core. In this manner, the plasmon resonance
would be geometrically tunable, such as by varying the thickness of
the shell layer. A disadvantage of this approach was its reliance
on bulk dielectric properties of the materials. Thus, thin metal
shells, with a thickness less than the mean free path of electrons
in the shell, were not described.
Despite the theoretical speculation, early efforts to confirm
tunability of the plasmon resonance were unsuccessful due to the
inability to make a particle having a metal shell on a dielectric
core with sufficient precision so as to have well-defined
geometrical properties. In these earlier methods, it was difficult
to achieve one or both of monodispersity of the dielectric core and
a well-defined controllable thickness of a metal shell, both
desirable properties for tuning the plasmon resonance. Thus,
attempts to produce particles having a plasmon resonance in keeping
with theoretical predictions tended to result instead in solutions
of those particles having broad, ill-defined absorption spectra. In
many instances this was because the methods of making the particles
failed to produce smooth uniform metal shells. Rather, the methods
tended to produce isotropic, non-uniform shells, for example shells
having a bumpy surface.
However, one of the present inventors co-developed a novel method
of making metallized nanoparticles (particles with a size between
about 1 nanometer and about 5 microns) that was successful in
producing metal-coated particles having narrow well-defined
spectra. Further, one of the present inventors co-discovered that
improved agreement with theoretical modeling of the metallized
nanoparticles resulted from the incorporation in the theory of a
non-bulk, size-dependent value of the electron mean free path. That
is, improved agreement with theory was achieved by developing an
improved theory applicable to thin metallization layers. Thus, in
the improved theory a dependence of the width of the plasmon
resonance on the thickness of the metallization layer was
described.
Particles having at least one substantially uniform metallization
layer have been termed metal nanoshells. Nanoshell structures that
exhibit structural tunability of optical resonance's from the
visible into the infrared can currently be fabricated. For example,
complete nanoparticle metallization shell layers containing gold
have been demonstrated. Gold has the advantage of a strong plasmon
resonance that can be tuned by varying the thickness of the
coating. More generally, the resonance may be tuned by varying
either the core thickness or the thickness of the coating, in turn
affecting the ratio of the thickness of the coating to the
thickness of the core. This ratio determines the wavelength of the
plasmon resonance. A further advantage of gold-coated particles is
that they have shown promise as materials with advantages in
imaging and diagnostics. In particular, they have utility as
band-pass optical filters, impeding the photo-oxidation of
conjugated polymers, and in conjunction with sensing devices based
on surface enhanced Raman substrates.
Present methods for making nanoshells involve purifying suspensions
of various intermediates, as well as purifying a suspension of the
nanoshell products. Methods of making nanoshells are disclosed, for
example, in U.S. Pat. No. 6,344,272 and in S. Oldenburg, R. D.
Averitt, S. Westcott, and N. J. Halas, "Nanoengineering of Optical
Resonances", Chemical Physics Letters 288, 243-247 (1998), each
hereby incorporated herein by reference. In particular, a method
for making nanoshells may include coating linkers onto substrate
particles so as to form linker-coated nanoparticles, seeding metal
colloids onto the linkers so as to form colloid-seeded
nanoparticles, and reducing metal onto the metal colloids so as to
form nanoshells. Further, at various stages, the method includes
purification of linker-coated nanoparticle suspensions,
purification of colloid-seeded nanoparticle suspensions, and
purification of nanoshell suspensions. Corresponding undesirable
byproducts correspondingly include, for example, excess linkers,
excess metal colloids, and excess metal ions, respectively.
Present methods for purifying nanoshells and intermediates thereof
rely on a technique for purifying product or intermediate thereof
that involves centrifugation and redispersal. The centrifugation
and redispersal may be repeated a suitable number of times to
achieve the desired level of purity. A disadvantage of this
conventional laboratory method is that it is difficult to scale up
to commercial scale. Thus, a scalable method of purifying
nanoshells and intermediates thereof is desirable.
Replacement of combined centrifugation and redispersal with
filtration is known to those skilled in the art of purification.
Filtration has the advantage that it is known to be a scalable
method of purification, in particular filtration may be scaled up
to commercial scale. However, previous laboratory scale attempts to
use filtration to separate excess colloid from colloid-seeded
nanoparticles serving as intermediates in making nanoshells have
been unsuccessful. This lack of success occurred despite the
typical size disparity between excess metal colloid, typically
between about 1 nm and 5 nm, and colloid-seeded nanoparticles,
typically between 50 nm and 5 .mu.m. In particular, attempts by the
present inventors to crossflow filter a suspension, as it results
from the seeding reaction, of colloid-seeded nanoparticles and
excess metal colloid using a filter having a 50 nm nominal pore
size have demonstrated ineffective passage of the excess colloids
through the filter in the filtrate. Rather, sufficient excess
colloids were found to remain in the retentate with the
colloid-seeded nanoparticles that subsequent reduction of metal so
as to form metal nanoshells was unsuccessful.
Thus, notwithstanding the above-described teachings, there remains
a need for a scalable method of making nanoshells, particularly
when the method involves intermediate colloid-seeded
nanoparticles.
SUMMARY OF THE INVENTION
According to a preferred embodiment, the present invention features
a method for purifying a suspension containing colloid-seeded
nanoparticles and excess colloids that includes adding to the
suspension a filter aid comprising a salt. The method further
includes filtering the suspension with a filter of a pore size
intermediate between the average colloid-seeded nanoparticle size
and the average excess colloid size, so as to form a retentate that
includes the majority of the colloid-seeded nanoparticles and a
filtrate that includes the majority of the excess colloids. Still
further, the method includes collecting the retentate. The salt is
preferably sodium chloride.
According to an alternative preferred embodiment, the present
invention features a method for making metallized nanoparticles
that includes providing a suspension of colloid-seeded particles,
where the colloids comprise a first metal, purifying the suspension
according to the above-described embodiment, and reducing ions of a
second metal onto the colloid-seeded particles so as to form the
metallized particles. The first metal may be any suitable metal
from which small colloids may be made. The first metal is
preferably gold. The second metal may be any suitable metal for
reduction. The second metal is preferably selected from among gold,
silver, copper, nickel, iron, palladium, and platinum.
According to any one of the above-described embodiments the present
invention preferably includes reduction of any tendency of the
presence of salt to impede reduction of metal onto the colloids
forming the colloid-seeded nanoparticles. Such a tendency may occur
when the colloid-seeded particles include free linkers remaining
from providing the colloid-seeded particles by binding metal
colloids to linkers coated on nanoparticles.
Thus, in one embodiment of the present invention, a method of
purifying a colloid-seeded nanoparticle includes following
salt-assisted filtration, with aqueous filtration to remove salt,
adding a protectant so as to passivate free linkers, and aqueous
filtration to remove excess protectant.
1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC)
is a preferred protectant. This embodiment has the advantage that
passivated colloid-seeded nanoparticles tend to be robust
precursors for subsequent metal reduction.
Further, in another embodiment of the present invention, a method
of purifying a colloid-seeded nanoparticle includes filtering using
a wash solution that includes a surfactant as well as the salt.
Polyethylene glycol is a preferred surfactant. This embodiment has
the advantage that the colloid-seeded nanoparticles suspending in
the salt-surfactant solution tend to be robust precursors for
subsequent metal reduction. Further, this embodiment has the
advantage that the colloid-seeded particles may remain in the
salt-surfactant solution for subsequent metal reduction without
further filtering.
Thus, the present invention comprises a combination of features and
advantages which enable it to overcome various problems of prior
methods. The various characteristics described above, as well as
other features, will be readily apparent to those skilled in the
art upon reading the following detailed description of the
preferred embodiments of the invention, and by referring to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more detailed description of the preferred embodiment of the
present invention, reference will now be made to the accompanying
drawings, wherein:
FIG. 1 is a flow diagram of a method for making metallized
nanoparticles according to an embodiment of the present
invention.
FIG. 2 is a schematic diagram of an embodiment of step 118 of FIG.
1.
FIG. 3 is schematic diagram of an alternative embodiment of step
118 of FIG. 1.
FIG. 4 is a schematic diagram of an embodiment of step 122 of FIG.
1.
FIG. 5 is a schematic diagram of an alternative embodiment of step
122 of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring initially to FIG. 1, according to a preferred embodiment
of the present invention, a method of making metallized
nanoparticles includes coating 110 nanoparticles with linkers,
filtering 112 the resulting linker-coated nanoparticles, seeding
114 linker-coated nanoparticles with metal colloids, adding 116
salt to the colloid-seeded nanoparticles, filtering 118 the
colloid-seeded nanoparticles, reducing 120 metal onto the
colloid-seeded nanoparticles, and filtering 122 the resulting
metallized particles. The filter preferably is of a pore size
intermediate between the average colloid-seeded nanoparticle size
and the average excess colloid size. Although the inventors do not
wish to be limited by this interpretation, it is believed that the
addition of salt has the advantage of reducing the hydrodynamic
radius of the excess colloids such that they are able to pass
through the pores in the filter.
The salt is preferably sodium chloride. Preferably salt is added to
a suspension of colloid-seeded nanoparticles and excess colloids to
give a salt concentration of between about 25 mM and 75 mM.
The filtration is preferably crossflow filtration. Crossflow
filtration conventionally includes a plurality of inner membranes
contained within an outer wall. The inner membranes are preferably
tubular, as is the outer wall. The inner membranes are preferably
arranged in a bundle within the outer wall. The inner membranes
include pores in their sides. Liquid contained between the outer
wall and the inner membranes is maintained at a different pressure
than liquid within the membranes. Thus there is a pressure
differential across the pores. The suspension to be separated is
fed into adjacent ends of the inner membranes. A pump propels the
suspension into the ends. As the suspension flows through the inner
tubular members filtrate passes though the pores. A wash solution
is pumped through the inner tubular membranes. The ratio of the
volume of wash solution to the volume of initial suspension is
preferably between about 4 and about 12. The filtrate contains the
solvent and byproducts. The retentate passes through the inner
membranes to their opposite ends where it is collected. The
retentate includes the particles being filtered. When cross-current
filtration is used to achieve filtration of nanoshells from
solution the retentate includes the nanoshells. Likewise, when
cross-current filtration is used to achieve separation of nanoshell
intermediates, such as linker-coated nanoparticles or
colloid-seeded nanoparticles, the retentate includes the nanoshell
intermediate.
Referring now to FIGS. 2 and 3, a first method and a second
alternative method of filtering a colloid-seeded particle
suspension are shown. While the methods are illustrated with
respect to step 118 of FIG. 1, it will be understood that the
methods are applicable more broadly to salt-assisted filtration of
colloid-seeded particles.
Referring now to FIGS. 4 and 5, a first method and a second
alternative method of filtering a metallized nanoparticle
suspension are shown. While the methods are illustrated with
respect to step 122 of FIG. 1, it will be understood that the
methods are applicable more broadly to filtration of metallized
nanoparticles, particularly metal nanoshells.
In some embodiments, the substrate particles are cores, preferably
monodisperse cores. Further, the cores are preferably silica cores.
Monodisperse silica cores are preferably grown using the Stober
method, described in Werner Stober, Arthur Fink, and Ernst Bohn, J.
Colloid and Interface Science 26, 62-69 (1968), entitled Controlled
Growth of Monodisperse Silica Spheres in the Micron Size Range,
hereby incorporated herein by reference. According to a preferred
embodiment, tetraethylorthosilicate (TEOS), ammonium hydroxide
(NH.sub.4 OH), and water are added to a glass beaker containing
ethanol, and the mixture is stirred overnight. The size of the
particles that result, herein termed Stober particles, is dependent
on the relative concentrations of the reactants.
According to some embodiments, variations in water, base
concentration, and TEOS concentration are used to produce
monodisperse silica spheres of various sizes. Temperature and
electrolyte concentration also affect the final diameter of the
particles. The following concentration ranges are preferred: 0.1 to
0.5 M TEOS, 0.5 to 17 M H.sub.2 O, and 0.5 to 3.0 M ammonia.
Further, a variety of alcohols may be used as solvents. Ethanol is
a preferred solvent. Higher ammonia concentrations provide larger
particles.
According to an exemplary procedure, uniform particles having a
diameter of 120 nm as measured by a transmission electron
microscope (TEM) may be prepared by the following method.
Approximately 50 milliliters (ml) of dry (100%) ethanol and 4 ml of
NH.sub.4 OH (25% NH.sub.3 in water), is stirred in a glass beaker.
To this solution, 2.2 ml of tetraethyl orthosilicate having a
purity of at least 99.999% is added and allowed to stir for at
least 8 hours. By varying the concentrations of NH.sub.4 OH, water
and silicate among other factors, the size of the silica particle
is varied from approximately 20 nm to 500 nm diameter. Larger core
particles are grown using a seeded growth technique where
additional TEOS and water were added to already formed silica
particles. Multiple additions of small amounts of additional
reactants allow monodisperse core particles to be grown as large as
5 microns.
The cores are preferably spherical particles between about 1
nanometers to about 5 microns in diameter, more preferably between
about 1 nanometers and about 4 microns in diameter. A plurality of
cores, for example in solution, is preferably monodisperse.
Monodisperse particles are defined herein as particles that have a
small variation in the distribution of particle sizes. For
spherical particles the size is given by the particle diameter. The
small variation is preferably quantified as the standard deviation.
In a preferred embodiment, core particles are characterized by a
distribution of diameters with a standard deviation of up to about
20%, more preferably about 10%.
It will be understood that variations in the above-described method
are contemplated. For example, it will be understood that the
substrate particles are not limited to core particles. A substrate
particle generally is any particle that includes at least an outer
surface of silica or other suitable substrate material. Further,
substrate particles may have shapes other than spherical. In
particular, although in preferred embodiments the core is spherical
in shape, the core may have other shapes such as cubical,
cylindrical, hemispherical, elliptical, and the like.
In some embodiments, alternative substrate materials may be used.
The substrate material preferably is characterized by a smaller
permittivity than the metal that is to be coated on it. Suitable
materials include dielectric materials and semiconducting
materials. Many dielectric materials are also semiconducting. In
particular, suitable substrate materials include silicon dioxide,
titanium dioxide, polymethyl methacrylate, polystyrene, gold
sulfide cadmium sulfide, cadmium sulfide, gallium arsenide, and the
like. Further, suitable substrate materials include dendrimers.
According to a preferred embodiment of the present invention,
linkers are coated onto core particles, thus initially
functionalizing the core particles for subsequent attachment of
metal colloids. When the core particles are silica, aminosilane
linkers are preferred. The silane group adsorbs to the silica
surface, and the amine group is exposed for further
functionalization. Thus, an advantage of the aminosiliane is that
it may function as a linker molecule, bridging a silica surface and
metal that may be attached to the amine group of the aminosilane.
3-aminopropyltrimethoxysilane (APTMS) is an exemplary silane. APTMS
is preferably added to a solution containing silica core particles.
Based on estimates, enough silane is preferably added to coat the
particles with multiple layers of silane. The solvent for the core
particles is preferably ethanol.
According to an exemplary embodiment, 10 ml of a silica particle
suspension such as prepared as described above, is added to a 50 ml
glass beaker. Next, pure aminopropyltriethoxy silane (APTES) is
added to the solution. For example, 40 microliters of undiluted
APTES is used for particles having diameters of 120 nm. The
solution is stirred for 2 hours, diluted to 200 mls and then heated
to a boil for four hours. Although the inventors do not wish to be
bound by this interpretation, it is believed that the heating step
promotes the reaction of silanol groups into Si--O--Si bonds and
strengthens the attachment of the silane to the silica.
In some embodiments, alternative linkers may be used. A linker
preferably is attachable to the core and has an atomic site that
has an affinity for a metal. The atomic site may be selected from
among sulfur, nitrogen, phosphorous, and the like. Further, the
linker molecule may include an amino acid that has a terminal group
that includes an active atomic site. Still further, when the core
includes active hydroxyl groups the linker is preferably a silane
that hydrolyzes in water to form hydroxyl groups that are bondable
to the active hydroxyl groups on the core. Suitable silanes include
APTMS, 3-aminopropyltriethoxysilane, diaminopropyl-diethoxy silane,
4-aminobutyldimethylmethoxy silane, mercaptopropyltrimethoxy
silane, diphenyltriethoxy silane reacted with tetrahydrothiophene,
and the like. Further, the linker molecule may be a non-metallic
material, such as CdS and CdSe, and the like.
In some embodiments, a linker may be a molecule cross-linked to
another linker molecule. Cross-linking may be achieved, for
example, by a thermal or a photo-induced chemical crosslinking
process.
According to a preferred embodiment of the present invention,
ultrasmall gold colloid (1-3 nm) is synthesized using a solution of
HAuCl.sub.4, NaOH, and tetrakis(hydroxymethyl)phosphonium chloride
(THPC) in water to produce "THPC gold". In an exemplary
preparation, gold colloid is synthesized using a solution of 45 mL
of water, 1.5 mL of 29.7 mM HAuCl.sub.4, 300 uL of 1M NaOH and 1 mL
(1.2 mL aqueous solution diluted to 100 mL with water) of
tetrakis(hydroxymethyl)phosphonium chloride (THPC).
According to an embodiment of the present invention, the THPC gold
colloid thus formed is preferably aged under refrigeration for
between about 5 and about 50 days, more preferably for between
about 14 and about 40 days.
According to an alternative, preferred, embodiment of the present
invention, the THPC gold colloid thus formed is filtered. The
filtering preferably occurs from between 0.5 and 2 hours after the
solution of HAuCl.sub.4, NaOH, and
tetrakis(hydroxymethyl)phosphonium chloride (THPC) is mixed. The
retentate may be discarded or, alternatively, dissolved for
recycling. The filtrate is collected and allowed to undergo further
reaction to form additional gold colloid. Preferably the filtrate
is maintained at room temperature (about 22.degree. C.) for 1 to 4
days after it is collected, allowing the further reaction to
proceed for 1 to 4 days. The gold colloid thus formed is termed
filtrate gold. While the inventors do not wish to be bound by this
interpretation, it is believed that filtrate gold colloid has a
more favorable charge distribution for facilitating nanoshell
production, in particular the process of reduction of metal ions
from solution onto the gold colloid.
According to still another preferred embodiment of the present
invention, gold colloid is then added to linker-coated silica core
particles. An aqueous solution of gold colloid is preferably added
to an ethanol solution of the silica core particles. The volume
ratio of the gold colloid solution to the silica particle solution
is preferably between 0.1:1 and 10:1. Exemplary volume ratios for
different size cores and a constant sized colloid are given in
Table 1, for an exemplary gold colloid concentration of 7.58E+014
and a core concentration of 1.00E+012, where E+n, denotes 10.sup.n
and leading zeros may be discarded in the exponent n. The combined
solution is preferably allowed to react overnight. The gold colloid
bonds to the amine-terminated silica particles which provide
nucleation sites for the chemical deposition of a metallic shell,
forming functionalized core particles. Thus, this completes the
functionalization of the core particles, thus forming gold
colloid-seeded nanoparticles. It will be understood that the core
particle may be any suitable alternative, such as described above,
to silica. Further, it is within the skill of one of ordinary skill
in the art to select a suitable linker according to the substrate
material forming the core particle.
TABLE 1 Min Vol (in Core Dia Core Surface Area 30% Area Gold Col
Gold Col Cross Section mL) Gold Col (nm) Rad(nm) (nm 2) (nm 2) Dia
(nm) Rad (nm) (nm 2) Gold Col/Core for 1 mL Core 50 25 7.85E+003
2.36E+003 3 1.5 7.07E+000 3.33E+002 4.40E+001 100 50 3.14E+004
9.42E+003 3 1.5 7.07E+000 1.33E+003 1.76E+000 150 75 7.07E+004
2.12E+004 3 1.5 7.07E+000 3.00E+003 3.96E+000 200 100 1.26E+005
3.77E+004 3 1.5 7.07E+000 5.33E+003 7.04E+000 250 125 1.96E+005
5.89E+004 3 1.5 7.07E+000 8.33E+003 1.10E+001 300 150 2.83E+005
8.48E+004 3 1.5 7.07E+000 1.20E+004 1.58E+001 350 175 3.85E+005
1.15E+005 3 1.5 7.07E+000 1.63E+004 2.15E+001 400 200 5.03E+005
1.51E+005 3 1.5 7.07E+000 2.13E+004 2.81E+001 450 225 6.36E+005
1.91E+005 3 1.5 7.07E+000 2.70E+004 3.56E+001 500 250 7.85E+005
2.36E+005 3 1.5 7.07E+000 3.33E+004 4.40E+001
According to some embodiments alternative metal colloids may be
used in place of gold colloids in attaching to a linker molecules.
Alternative metals include silver, platinum, tin, and nickel.
Reduction of shell metal preferably includes included mixing a
functionalized dielectric substrate, a plurality of metal ions, and
a reducing agent, in solution. Formaldehyde is a preferred reducing
agent. In some embodiments, the metal includes at least one element
selected from the Periodic Table of the Elements that are commonly
known as metals. As used herein, metals include those elements
disclosed in the USPTO Manual of Classification as metals. Both the
old IUPAC notation, with Roman numerals, and the new notation, with
Arabic numbers will be used herein. See, for example Lewis, Richard
J., Sr., "Hawley's Condensed Chemical Dictionary" (1997, John Wiley
and Sons), the inside front cover page, hereby incorporated herein
by reference, for a comparison of notations. In particular, Group I
metals include Group 1 metals (Li, Na, K, Rb, Ca, and Fr) and Group
11 metals (Cu, Ag, and Au). Group II metals include Group 2 metals
(Be, MG, Ca, Sr, Ba, and Ra) and Group 12 metals (Zn, Cd, and Hg).
Group III metals include Group 3 metals (Sc and Y) and Group 13
metals (Al, Ga, In, and Tl). Group IV metals include Group 4 metals
(Ti, Zr, and Hf) and Group 14 metals (Ge, Sn, and Pb). Group V
metals include Group 5 metals (V, Nb, and Ta) and Group 15 metals
(As, Sb, and Bi). Group VI metals include Group 6 metals (Cr, Mo,
and W) and Group 16 metals (Po). Group VII metals include Group 7
metals (Mn, To, and Re). Group VIII metals include Group 8 metals
(Re, Ru, and Os), Group 9 metals (Co, Rh, and Ir), and Group 10
metals (Ni, Pd, and Pt). A metallic material forming shell 16
preferably is selected from the elements of Groups I and VII. More
preferably, the metallic material is selected from among copper
(Cu), silver (Ag), gold (Au), nickel (Ni), platinum (Pt), palladium
(Pd), and iron (Fe). The metal may include primarily a single
element. Alternatively, the metal may be a combination of at least
two elements, such as an alloy, for example a binary alloy. Yet
alternatively, in some embodiments, the metal is a synthetic metal.
A synthetic metal is defined herein as an organic or organometallic
material that has at least one characteristic property in common
with a metal. For example, the property may be electrical
conductivity. Thus, synthetic metals include conducting polymers,
such as polyacetylene, polyanaline, and the like. Therefore,
suitable metals include any of an elemental metal, an alloy, a
synthetic metal, and the like, and combinations thereof.
When the metal is selected from among silver, copper, and nickel,
as disclosed in U.S. Utilily application Ser. No. 09/966,544, filed
Sep. 27, 2001, the method preferably further includes raising the
pH of the solution effective to coat the substrate with the metal.
In particular, in one embodiment, as disclosed therein,
gold-functionalized silica particles are mixed with 0.15 mM
solution of fresh silver nitrate and stirred vigorously. A small
amount (typically 25-50 micro-liters) of 37% formaldehyde is added
to begin the reduction of the silver ions onto the gold particles
on the surface of the silica. This step is followed by the addition
of doubly distilled ammonium hydroxide (typically 50 micro-liters).
The "amounts" or "relative amounts" of gold-functionalized silica
and silver nitrate dictate the core to shell ratio and hence the
absorbance.
The above-described preferred embodiments may be combined in any
suitable combination.
Thus, for example, Applicants have discovered a method for
purifying a suspension containing colloid-seeded nanoparticles and
excess colloids that includes:
(a) adding to the suspension a filter aid comprising a salt;
(b) filtering the suspension with a filter of a pore size
intermediate between the average colloid-seeded nanoparticle size
and the average excess colloid size, so as to form a retentate
comprising the majority of the colloid-seeded nanoparticles and a
filtrate comprising the majority of the excess colloids; and
(c) collecting the retentate.
The salt may include sodium chloride. Step (b) may include:
(b1) loading the suspension to a retentate zone, wherein the filter
separates the retentate zone from a filtrate zone; and
(b2) passing a first wash liquid through the retentate zone.
Step (b2) may include passing the first wash liquid tangentially to
the filter such that the filtering comprises crossflow filtering.
The first wash liquid may include a solution of the salt added in
step (a), the solution having a salt concentration about equal to
the salt concentration of the suspension after step (a). The first
wash liquid may further include a surfactant. The surfactant may be
polyethylene glycol. Step (b) may further include:
(b3) passing a second wash liquid through the retentate zone.
The second wash liquid may include water.
Each colloid-seeded nanoparticle may include:
a plurality of linkers; and
a plurality of colloids bound to a portion of the linkers, the
remaining portion of the linkers being free; and
wherein the method further comprises:
(d) adding a plurality of protectants to the retentate so as to
passivate the free linkers.
Each linker may include an amine moiety and wherein each protectant
comprises 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride (EDC).
Further, Applicants have discovered a method for treating a
suspension containing colloid-seeded nanoparticles and excess metal
colloids that includes:
(a) adding to the suspension a filter aid comprising a salt;
(b) filtering the suspension with a filter of a pore size
intermediate between the average seeded nanoparticle size and the
average excess colloid size, so as to form a retentate comprising
the majority of the precursors and a filtrate comprising the
majority of the excess colloids, wherein the filtering comprises:
(b1) loading the suspension to a crossflow filtration zone
containing the filter; and (b2) passing a first wash liquid through
the filtration zone tangentially to the filter; and
(c) collecting the retentate.
Each metal colloid may include gold. Each gold colloid may include
filtrate gold. The method may further include:
(d) reducing metal ions onto the seeded nanoparticles so as to form
metallized nanoparticles. The metal ions may be selected from the
group consisting of gold ions, silver ions, platinum ions,
palladium ions, nickel ions, iron ions, and copper ions.
Still further, Applicants have discovered a method for making
metallized nanoparticles via intermediate colloid-seeded
nanoparticles, where the method includes:
(a) providing a suspension of the colloid-seeded nanoparticles,
wherein the colloids comprise a first metal;
(b) adding to the suspension a filter aid comprising a salt;
(c) filtering the suspension with a filter of a pore size
intermediate between the average seeded nanoparticle size and the
average excess colloid size, so as to form a retentate comprising
the majority of the seeded nanoparticles and a filtrate comprising
the majority of the excess colloids;
(d) collecting the retentate; and
(e) reducing ions of a second metal onto the colloid-seeded
nanoparticles so as to form the metallized nanoparticles.
The salt may include sodium chloride. Step (b) may include:
(b1) loading the suspension to a retentate zone, wherein the filter
separates the retentate zone from a filtrate zone; and
(b2) passing a first wash liquid through the retentate zone.
Step (b2) may include passing the first wash liquid tangentially to
the filter such that the filtering comprises crossflow filtering.
The first wash liquid may include a solution of the salt added in
step (a), the solution having a salt concentration about equal to
the salt concentration of the suspension after step (a). The first
wash liquid may further include a surfactant. The surfactant may be
polyethylene glycol. Step (b) may further include:
(b3) passing a second wash liquid through the retentate zone.
The second wash liquid may include water. Step (a) may include
mixing a suspension of colloids with a suspension of linker-coated
nanoparticles so as to form the suspension of the colloid-seeded
nanoparticles. The colloid-seeded nanoparticles may include free
linkers and the method may further include:
(d) adding a plurality of protectants to the retentate so as to
passivate the free linkers.
Each linker may include an amine moiety and each protectant
comprises 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide
hydrochloride (EDC). The second metal may have the same identity as
the first metal. The second metal may differ from the first metal.
The first metal may include gold. The colloids may include filtrate
gold. The second metal may be selected from among gold, silver,
platinum, palladium, nickel, iron, and copper.
Without further elaboration, it is believed that one skilled in the
art can, using the description herein, utilize the present
invention to its fullest extent. The following embodiments are to
be construed as illustrative, and not as constraining the scope of
the present invention in any way whatsoever.
EXAMPLES
Gold Metallization of Silica Nanoparticles
As a general guide, using any of the following exemplary
procedures, 1 mL of silica particles will produce about 250 mg of
shells (this will vary depending on shell thickness, silica
concentration and silica size.)
Example 1
Salt-Assisted Filtration Followed by Passivation of Free
Linkers
I) Preparation of `Filtrate Gold Colloid` 1) Make THPC Gold Colloid
by the standard method, such as disclosed in U.S. Pat. No.
6,344,272, and in S. Oldenburg, R. D. Averitt, S. Westcott, and N.
J. Halas, "Nanoengineering of Optical Resonances", Chemical Physics
Letters 288, 243-247 (1998), which each incorporated herein by
reference. 2) Concentrate the Gold Colloid solution with the CFF
(400 KDa filter) within 1 hour of synthesis and collect the
filtrate in a clean vessel (the concentrated gold colloid can be
discarded.) 3) Allow the filtrate to age at room temp for 48-72
hours and then use or refrigerate (the filtrate solution will have
reacted to form `filtrate gold colloid`.)
II) Preparation of Seed Solution 1) Make monodisperse APTES
functionalized silica particles by a standard method (e.g. Stober
synthesis, commercial particles, etc.) such a method as disclosed
herein. The silica particles should be about 5 wt % in EtOH--about
1E12 particles/mL for 100-400 nm diameter particles. (Following the
original protocol, such as disclosed in U.S. patent application
Ser. No. 09/038,377, filed Mar. 11, 1998, and in S. Oldenburg, R.
D. Averitt, S. Westcott, and N. J. Halas, "Nanoengineering of
Optical Resonances", Chemical Physics Letters 288, 243-247 (1998),
which each incorporated herein by reference, for shell synthesis
will produce silica particles in this concentration range.) 2) Add
100 mL of filtrate gold colloid per 1 mL of silica particles used.
(Filtrate gold conc. is about 5E14 colloid/mL as made.) 3) Add 575
mg of NaCl per mL of silica used (This will give a 50 mM salt
conc.) 4) Allow the seed solution to stir for 15 min to 1 hour.
III) Diafiltration of Seed Solution 1) Prepare wash solutions: A)
Salt Solution: 5 L DI water+9 g NaCl (50 mM NaCl) B) Water: 5 L DI
water 2) Add the seed solution to the CFF vessel (50 nm pore
filter) 3) Add Salt solution until the CFF pump is primed 4) Turn
on the pump and concentrate the Seed Solution until the total
volume in the CFF is approximately 400 mL. (In order to remove all
permiate at least 8 times this volume must be diafiltered through
the system. So for a 400 mL volume in the CFF, at least 3200 mL of
wash solution must be used. Note for larger batches make larger
wash solutions based on the 8.times.factor.) 5) Diafilter the Salt
Solution. This will remove all the excess gold colloid from the
seed particles. Backflush frequently to make sure a cake does not
form on the filter (a filter cake could prevent the gold colloid
from passing though the filter.) 6) After the Salt Solution,
diafilter with water to remove the salt. 7) Concentrate the volume
in the CFF to as little as possible without sucking air into the
pump. 8) Open the relief valve and drain the liquid from the CFF
into a clean bottle. 9) Most of the seed will have formed a cake on
the filter (even with backflushing.) To recover this seed, hook the
filtrate outlet line up to the DI water line and backwash by
flowing clean DI water backwards through the filter. This will
flush the seeds from the filter and allow recovery through the
relief valve and into the collection bottle. Try to minimize the
amount of DI water used so as not to dilute the seed more than is
necessary. 10) Clean the CFF and store it with water inside the
filter cartridge.
IV) EDC Treatment 1) Too the seed solution add 125 uL of
concentrated acetic acid for each 100 mL of seed solution. 2) Add
125 mg of EDC for each 100 mL of seed solution. 3) The pH should be
about 5. If not adjust the pH with HCl or NaOH but do not use any
acid or base with an amine or carboxylic acid group. 4) Allow the
seed solution to stir for 20 min to 2 hours and then diafilter (see
next step)
V) Diafilter EDC/Seed 1) Diafilter the Seed solution to remove
excess EDC and acetic acid. Use the same procedure as described
above but only filter with the water wash solution.
VI) Seed Dilution 1) The seed should be diluted to a concentration
of approximately 1E9 particles/mL
VII) Shell Growth 1) Grow shells in the standard way (i.e. use
sweeps to find the proper ratio of Kcarb to seed and then scale up
to a large batch--use formaldehyde as the reducing agent.)
VIII) Diafiltration of Shells 1) Follow the same procedure as in
diafiltration of seed solution above except substitute the
following wash solutions: A) 5 L DI, 9 g NaCl, 30 g
tris(hydroxymethyl)aminomethane (to remove any gold colloid that
may have formed during shell growth. If no gold colloid is present
just filter with solution B.) B) One of the following depending on
the final form the shells should be in: i) 5 L DI and 30 g tris ii)
5 L EtOH (if using EtOH backwash with Ethanol not DI H2O) iii) 5 L
DI and appropriate surfactant or PEG (1 mM concentration is usually
good)
IIX) Optional Thiol Capping 1) Diafilter shells with EtOH as
describes above. 2) Add alkane thiol to shells and allow to react
overnight. (Amount should be calculated based off of number of
shells and shell surface area. Using excess is ok.) 3) Diafilter as
describes previously using the solvent that you want the shell to
be suspended in.
Example 2
Salt-Assisted Filtration with Salt-Surfactant Wash Solution I)
Follow steps I and II in Example 1. II) In step III of Example 1
use the following wash solution: 5 L DI, 9 g NaCl and 50 g 10,000
MW PEG (1 mM.) Do not filter with the second wash solution (i.e.
The seeds are left in NaCl/PEG solution after diafiltration.) III)
Skip steps IV and V in Example 1 IV) Follow steps VI through IIX in
Example 1.
U.S. Provisional application Ser. No. 60/259,757, filed Jan. 4,
2001, entitled "Method for Large Scale Production of Nanoshells",
is hereby incorporated herein by reference.
While preferred embodiments of this invention have been shown and
described, modifications thereof can be made by one skilled in the
art without departing from the spirit or teaching of this
invention. The embodiments described herein are exemplary only and
are not limiting. Many variations and modifications of the method
are possible and are within the scope of the invention. For
example, unless indicted otherwise, method steps may be carried out
simultaneously or sequentially in any order. Further, unless
indicated otherwise method steps may be carried out in any order.
Accordingly, the scope of protection is not limited to the
embodiments described herein, but is only limited by the claims
that follow, the scope of which shall include all equivalents of
the subject matter of the claims.
* * * * *